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Compensation Plans (Details) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 USD ($) shares | Mar. 31, 2017 USD ($)
Performance Shares [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 3 years |
Stock-based compensation expense | $ 2.4 | $ 2.5
Unrecognized stock based compensation expense | $ 15.6 |
Performance Shares [Member] | Minimum [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Multiplier applied to PSU awards at settlement | 0 |
Performance Shares [Member] | Maximum [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Multiplier applied to PSU awards at settlement | 2 |
Restricted Stock Units (RSUs) [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Stock-based compensation expense | $ 2.7 | $ 2.5
Unrecognized stock based compensation expense | $ 16.2 |
Number of shares represented by each RSU | shares | 1 |

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